This and Prisoner of Azkaaban are, so far, the best books of the series. Harry's development and maturity as well as flaws show clearly but are clearly interrelated. This makes for a good, well-rounded character. Dolores Umbridge was the sociopathic villain that the series needed.
I do like how Rowling has kept the mystery around why Voldemort needs Harry and how it played out. I felt a great deal of pity for Snape and can absolutely understand why he hates James Potter. However, not completely. Never hold the sins of the father against the son.
What I liked most here is the limited use of Malfoy and the fact that despite Potter being in a much worse place than where he was in Book 4 (at least half the school thinking he was an attention-seeking lunatic, being banned from Quidditch, the existence of Dolores Umbridge, and his final blow in the last book) Rowling didn't go over-the-top with Malfoy twisting the knife some more.
So obviously, this means Malfoy will be heavily featured in the next me. Great...